Solvate Ionic Liquids based on branched glymes enabling high performance lithium metal batteries
Gabriele Lingua, Guillaume Depraetère, Jun Wang, Jason E. Bara, Maria Forsyth, David Mecerreyes
Abstract
The development of electrolytes is crucial to obtain lithium metal batteries with improved safety, low toxicity and stability. In this context, the successful formation of solvate ionic liquids (SILs), essentially “quasi” liquid salts of lithium, achieved by mixing linear glymes and lithium salt, has been reported as a high-performance electrolytes. However, linear glymes such as di-tri or tetraglyme offer limited opportunities for tailoring composition and are considered as toxic chemicals. As an alternative, designing new branched glymes using glycerol as a skeleton offers opportunities for molecular design and the potential for lower toxicity than linear analogs. Herein, several branched glymes, are investigated in combination with equimolar amount of lithium bis(fluorosulfonyl)imide (LiFSI) salt, to obtain a new family of SILs. The obtained electrolytes showed high ionic conductivity (up to 10 -4 to 10 -3 S cm −1 at 25 °C ) and low glass transition temperature, without melting and crystallization peaks. The superior chelating ability towards of fluorinated glymes, along with the formation of specific interactions allowed to obtain SILs with extended oxidation stability ( ∼ 5 V vs ). All SILs showed long and stable cycling in lithium metal cell against LFP cathodes at 25 °C with high retention capacity of ∼ 98 % . Furthermore, Li/NMC622 cells with fluorine-based SILs shows superior stability at elevated upper cut-off voltages (up to 4.5 V vs ) in comparison with other electrolytes, delivering 180 mA h g −1 at 25 °C , with Coulombic Efficiency approaching 100 % . • Novel Solvate Ionic Liquids (SILs) were prepared using less toxic branched glymes. • The electrolytes work stably in Li-metal cell in combination with LFP and NMC622. • Stable specific capacity output of 150 mAh g −1 at C/10 at RT with NMC622. • Fluorinated-based SILs stably work at 4.5 V vs Li + /Li upper cut-of potential. • The chemistry of the glymes affects the electrolyte/electrode interface.